A conventional signal-processing circuit arrangement is described in a publication by the firm Interlink Electronics Europe, Echternach, LU (see information pertaining to FSR.TM.-SBE sensor interface).
The characteristics and problems of a basic circuit of a signal-processing circuit arrangement described in the above publication are shown in FIGS. 4 and 5.
In FIG. 4, which shows a simplified, electrical equivalent circuit diagram of an FSR sensor, a pressure-variable foil resistor Rp is disposed between a first and second terminal 1,2, in parallel to a series connection of a diode D to a series resistor Rs. When, in response to a pressure-loaded state of the FSR foil, diode D is reverse-biased, its foil resistor Rp is able to be defined within a range of between 1 k.OMEGA. and approximately 40 k.OMEGA., depending on the size and construction type of the same. However, when the FSR foil is not in a pressure-loaded state, the value of this resistor Rp lies in a range above 60 k.OMEGA.. When diode D is reverse-biased, a short-line fault of the FSR foil can also be detected, namely, when the resistance value of resistor Rp between input terminals 1 and 2 lies below approximately 0.5 k.OMEGA..
When diode D is forward-biased, a line interruption can be detected, since the main portion of the current then flows through resistor Rs, whose resistance value lies between 200.OMEGA. and 5 k.OMEGA..
The above description of FIG. 4 make it clear that an FSR foil having such properties is exceptionally suitable for use in a seat-occupancy detection device in mctor vehicles when such an FSR foil is arranged, e.g., between the seat cover and a rubberized-hair mat of the seat (described in German Patent No. 42 37 072).
The above-described publication by the firm Interlink Electronics Europe includes a signal-processing circuit arrangement as shown in FIG. 5, which is suitable for evaluating the above-mentioned FSR-foil states with the aid of a microcontroller unit MCU. A first and a second external transistor T.sub.1, T.sub.2 are switchable on and off by two ports Port.sub.0, Port.sub.1 of the MCU. These transistors T.sub.1, T.sub.2, together with reference-voltage sources V.sub.Ref+ and collector resistors R.sub.H, R.sub.L, in each case form a switchable voltage source which, with the aid of a low-pass filter TP1, TP2 for limiting interference, allows currents of different directions to flow through FSR-foil pressure sensor FSR, so that, depending on the polarity of the diode contained in FSR-foil pressure sensor FSR in conducting direction or blocking director, and depending on the status of FSR-foil pressure sensor FSF, different currents flow through FSR-foil pressure sensor FSR which are able to be sampled in the form of voltage signals at analog-digital converter inputs ADC.sub.0, ADC.sub.1 of microcontroller unit MCU. Microcontroller unit MCU samples the sensor states, program-controlled, in sequential phase sections. In the above-described, conventional signal-processing circuit arrangement shown in FIG. 5, the switching instants of external transistors T.sub.1, T.sub.2 cannot be brought exactly into agreement with the switching instants of sampling inputs ADC.sub.0 and ADC.sub.1 of the MCU. Disadvantageously, the known circuit arrangement also needs two analog-digital converter input ports ADC.sub.0 and ADC.sub.1, as well as two independent ports Port.sub.0 and Port.sub.1 of the MCU for detecting the status of an FSR foil. Since the discrete external transistors T.sub.1 and T.sub.2 have no "matching" behavior, because of tolerances occurring, the states of the FSR foil must be determined by a costly differential measurement. Disadvantageously, two of the analog-digital converter ports of microcontroller unit MCU, which are always scarce anyway, are needed per FSR foil to be evaluated.